RAM

From TheBestCaseScenario

Ram, or Random access memory, is the short term storage of your computer. It acts as a buffer between the slower hard disk, and the CPU, by adding more ram to a computer, you gain the ability to increase the computers speed through a process known as caching. When the memory has extra space the operating system will load commonly used files from the hard drive, to the ram. This gives the benefit of actually using the ram that is inside of your PC, as well as giving faster program opening times and better responsiveness. The more ram you have the quicker your PC will seem up to a point. 4GB is good for most PC users, including gaming Pc's but if you have 6Gb or 8gb you will notice slight increases in performance in many applications.

Ram comes in many flavors, but the most common today are

DDR3, which is the current standard for the latest processors. DDR2, which is the former standard for processors from the AM2 and Socket 775 era of processors. DDR, An older standard, but still commonly found in some slightly older pc's. It tends to be more expensive than ddr3 and ddr2 because it is produced in smaller quantities.

If you are thinking of building a new PC, go with DDR3 ram, as it is the most abundantly available at this time, and is also the current standard, you can still use DDR2 in many applications, but I would recommend using the newer standard to future proof your new PC.